This invention relates to new and improved network structures and methods for making such network structures, and particularly to network structures and methods for making them by embossing or forming ribs in both sides of a thermoplastic polymeric sheet in a particular manner so as to permit spontaneous fibrillation or opening of the network structure upon drawing in one direction or in two preferably perpendicular directions and to provide a uniform open network structure having desirable strength characteristics.
In the manufacture of networks, it has previously been proposed to form continuous diagonal grooves in one direction in one side of a sheet of plastic material and continuous diagonal grooves in the opposite direction on the other side of the sheet so that upon subjecting the sheet to biaxial stretching the thin parts of the sheet, at the crossing points of the grooves, split and form perforations thereby opening the material into a network. For example, see U.S. Pat. No. 3,488,415 to A. G. Patchell et al. The networks therein disclosed are formed in such a manner as to have thicker masses at the points where the ridges cross, which behave as discrete areas of reinforcement, since on biaxial stretching or drawing of the embossed sheet the thick areas where the ridges cross orient only to a limited extent if at all. The tensile strength and tear characteristics of such a network are relatively poor because the presence of the unoriented thick areas weakens the tensile strength and tear resistance of the network so prepared, and such a network is not uniform in appearance. U.S. Pat. No. 3,500,627 to Charles W. Kim discloses making yarn by forming on one side of a ribbon of plastic material a plurality of parallel filament forming ribs and on the other side a plurality of fibril forming cross-ribs arranged at an acute angle to the filament forming ribs. The ribbon is then uniaxially oriented and mechanically fibrillated by means of a toothed fibrillating device to break the fibril forming ribs and form a yarn having fibrils extending laterally therefrom. Use of mechanical fibrillation makes reproducing uniform network structures very difficult.